This paper investigates the hydrodynamical aspects of a floating two body system. The emphasis is geared to a side by side mooring of a FPSO vessel and a LNG carrier. The main focus is on the following issues: ¿ Determination of the mean and low frequency wave drift forces for multi-body systems * Development of a robust linear potential solver for multibody systems In the paper a lid technique will be presented to circumvent unrealistic high water velocities on the ship's hull. Also the accurate integration of the complete Green's function will be highlighted. Introduction In the present-day development of offshore activities there is an increasing interest in the behavior of multi-body systems. One example of such a system is a floating oil or gas production annex storage facility, to which an export tanker is moored during loading operations. When evaluating the feasibility of such a system, attention will be focussed on the motions of both the production platform and the tanker and the forces in the moorings and the relative motions of loading arms. Interaction effects will be present in the second order wave drift forces. The narrow spacing between the two adjacent vessels is an added complexity, both with respect to wave loading and dynamical response. A frequently used option in relation to multiple body computations has so far been to use single body hydrodynamics input, i.e. to ignore the effects of hydrodynamical interaction between the different bodies. The more general problem of how to establish convergence for hydrodynamical coefficients for multiple body systems is also discussed throughout the paper. Particular emphasis is put on low fiequency excitation forces, which for multiple bodies have to be computed by integration of the local body pressure.

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